Tracking loops are used to track a phase, a position, an angle, etc., and provide as an output an estimate of this input phase, position, angle, etc. In many situations the input signals of the tracking loop are noticeably affected by offset. For instance, when the input signals are obtained from magnetic field sensing elements, they typically have an offset-related component, i.e. a component not representative of the magnetic field. Some sensing elements, e.g. horizontal and vertical Hall elements, typically have a significant offset with e.g. a magnitude comparable to or larger than the useful (magnetic-field related) signal component. Each input signal typically has an offset distinct from the offset of other input signals, but offsets may be correlated, e.g. when common causes such as stress and/or processing gradients affect the offsets. Also, front-end circuits, such as (low-noise) amplifiers, may have offsets.
Without measures being taken, the tracking loop accuracy may be severely affected by the offsets. It is known in the art that chopping may be used for separating (in frequency) offset-related components from the useful input signal components. To accomplish this, chopping makes use of modulation, e.g. with a square wave having symmetric values ±1. By separating the harmful offsets from the useful signals (in frequency), frequency selective methods, such as filtering, can be applied to reduce or eliminate the impact of offset.
Chopping is for instance applied in U.S. Pat. No. 7,714,757, disclosing a chopper-stabilized ADC comprising a modulator to modulate an amplitude of an analog input signal at a frequency to produce a modulated input signal, a mixer amplifier that amplifies the modulated input signal to produce an amplified signal and a demodulator to demodulate the amplified signal at that frequency to produce a demodulated signal.
Chopping is also applied in U.S. Pat. No. 9,685,967, disclosing a Sigma-Delta ADC comprising a forward path with analog chopper circuitry. Chopping may be accomplished by modulation with an analog carrier, e.g. cos(2πfct) with fc a chopping frequency. In induction-based position sensors the chopping operation may be due to the modulation caused by an excitation signal applied to an excitation coil.
It is known in the art that chopped output signals can be obtained from a Hall element (HE) by means of spinning (also referred to as current spinning). In this case the contacts of a Hall element are biased and read out in different arrangements. These arrangements are obtained by switching biasing and readout to particular contacts. A result of a particular type of spinning is that the differential signal appearing at the spinning readout switches comprises a low-frequency (quasi DC) offset component and a signal component responsive to a magnetic field up-modulated to higher frequencies according to a chopping frequency related to the spinning frequency (i.e. the frequency with which one cycles from one spinning phase to the next). As a result of a particular spinning scheme, the Hall element signals may, for instance, be representable asHEk(θk)=c(n)ASk(θk)+OK k=0,1, . . . ,N−1  (1)wherein Sk is a signal representative of an input signal of the tracking loop, for instance represented as
                                                        S              k                        ⁡                          (                              θ                i                            )                                =                                                    cos                ⁡                                  (                                                            θ                      i                                        -                                          k                      ⁢                                                                        2                          ⁢                                                                                                          ⁢                          π                                                N                                                                              )                                            ⁢                                                          ⁢              k                        =            0                          ,        1        ,                                  ⁢                  .                                          .                                          .                ⁢                                  ,                  N          -          1                                    (        2        )            and wherein c(n)=(−1)n denotes the modulation function associated with the spinning scheme, A denotes an amplitude of the Hall element signals and Ok represents an offset of the kth sensing element. The integer n is used for referring to the nth time slot during which sensor signals are being read out. An advantage of this particular spinning scheme is that the offset Ok in expression (1) may combine with an offset of a front-end circuit that follows thereafter, both offsets being essentially at DC. Therefore, the effect of offset of the front-end circuits can be taken into account by considering the Ok to be the combination of a Hall-element offset plus an input-referred offset of the front-end circuit. Therefore, also the front-end related offset sources can be handled by the same spinning/chopping scheme applied for readout of the Hall elements.
In the abovementioned examples, at some later point in the signal chain, a second chopper is applied for demodulating the useful signal to baseband and at the same time up-modulating the unwanted offset contributions to higher frequencies. When the modulation function assumes only the values ±1, demodulation can be accomplished using the same modulation function c(n) associated with the spinning scheme.
It is not required to apply the same spinning scheme to the different Hall elements. For instance, when one Hall plate cycles through a sequence of spinning phases (e.g. 0, 1, 2, 3, . . . ) another Hall plate may cycle through a delayed or advanced sequence (e.g. 2, 3, 0, 1, . . . ). Also, the direction of cycling through the spinning phases can be altered (e.g. 2, 1, 0, 3, . . . ).
In classical angle architectures offsets on sensed signals may be removed prior to an angle calculation. Such a solution cannot be easily applied in a tracking loop, especially when many sensing signals are involved.
Hence, there is a need for an offset compensation circuit that is suited for use in a tracking loop.